Python: How to match nested parentheses with regex?

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As others have mentioned, regular expressions are not the way to go for nested constructs. I’ll give a basic example using pyparsing:

import pyparsing # make sure you have this installed

thecontent = pyparsing.Word(pyparsing.alphanums) | '+' | '-'
parens     = pyparsing.nestedExpr( '(', ')', content=thecontent)

Here’s a usage example:

>>> parens.parseString("((a + b) + c)")

Output:

(                          # all of str
 [
  (                        # ((a + b) + c)
   [
    (                      #  (a + b)
     ['a', '+', 'b'], {}   
    ),                     #  (a + b)      [closed]
    '+',
    'c'
   ], {}
  )                        # ((a + b) + c) [closed]
 ], {}  
)                          # all of str    [closed]

(With newlining/indenting/comments done manually)

Edit: Modified to eliminate unnecessary Forward, as per Paul McGuire’s suggestions.

To get the output in nested list format:

res = parens.parseString("((12 + 2) + 3)")
res.asList()

Output:

[[['12', '+', '2'], '+', '3']]

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